The Hydrogen Halides

Question 1

What are the hydrogen halides?

Answer 1

HF, Hydrogen fluoride
HCl, hydrogen chloride
HBr, hydrogen bromide
HI, hydrogen iodide

Question 2

What are the properties of the hydrogen halides?

Answer 2

• all gases
• made up from covalent molecules
• dissolve in water, to give H+ and X- (acidic solutions)

Question 3

What is the equation of making hydrogen halides?

Answer 3

NaX (s) + H2SO4 (aq) => NaHSO4 (aq) + HX (g)

Question 4

What colour fumes do the hydrogen halides give off when reacted with conc sulphuric acid?

Answer 4

White misty fumes of the halide

Question 5

What is the trend down the halide ions?

Answer 5

• more reactive down the group
• better at taking the extra electron, them themselves getting oxidised
• Br- and I- are both powerful enough to become reducing agents to reduce sulphuric acid so an alternative reaction takes place

Question 6

What is the equation for Br- reaction with sulphuric acid?

Answer 6

2H+ (aq) + 2Br- (aq) + H2SO4 => SO2 (g) + Br2 (l) + 2H2O (l)
SO4 2- has been reduced
Br- has been oxidised

Question 7

What is the equation for I- and sulphuric acid?

Answer 7

H+ (aq) + In (aq) + H2SO4 (aq) => H2S (g) + I2 (s) + H2O (l)

Question 8

Thermal stability of hydrogen halides

Answer 8

• going down group 7 the strength of the H-X bond decreases (halogen atom gets bigger so bonding electrons are further from the nucleus ), **so the thermal stability of HX decreases going down the group.

Question 9

What happens to the hydrogen halides on heating?

Answer 9

• HF and HCl are not broken down
• some HBr is broken down giving brown fumes of Br gas
  2HBr (g) => H2(g) + Br2 (g)
• Most HI breaks down giving large quantities of purple iodine vapour
  2HI (g) => Hw (g) + I2 (g)

Question 10

What is the trend for acidity in the hydrogen halides?

Answer 10

All the hydrogen halides dissolve well in water giving acidic solutions. HCl, HBr and HI fully ionise when in aqueous solutions so they are strong acids

Question 11

What is the trend for reacting hydrogen halides with ammonia?

Answer 11

All the hydrogen halides react and ammonium salt is formed as tiny white particles which look like a white cloud (a good test for hydrogen halides)

Question 12

Trend for reaction of hydrogen halides with conc sulphuric acid

Answer 12

• HF and HCl do not react
• HBr is a powerful enough reducing agent to reduce H2SO4 to SO2
• HI is a powerful enough reducing agent to reduce H2SO4 to H2S